Low Temperature Air Fractionation with External Fluid

ABSTRACT

In low-temperature air fractionation, feed air ( 8 ) is cooled in a main heat exchanger ( 9 ) and introduced into a distillation column system for nitrogen-oxygen separation ( 11, 43 ), which system has at least one single column ( 12 ). At least one nitrogen-enriched or oxygen-enriched product stream ( 15, 16, 17; 53; 51, 56, 57; 19, 60 ) is withdrawn from the distillation column system for nitrogen-oxygen separation and warmed in the main heat exchanger ( 9 ). A fluid from an external source is passed at least at times into a liquid tank ( 70 ). At least at times fluid ( 71 ) is withdrawn in the liquid state from the liquid tank ( 70 ), vaporized in the main heat exchanger ( 9 ) and is obtained as gaseous additional product ( 72, 73 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to concurrently filed application “Process And Device For Low Temperature Air Fractionation” by Stefan Lochner, Attorney Docket No. LINDE-0677, claiming priority of DE 102007051184.3 filed Oct. 25, 2007, incorporated by reference herein.

SPECIFICATION

Processes and devices for low temperature fractionation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [Cryogenic engineering], 2nd edition 1985, chapter 4 (pages 281 to 337). Examples of relevant air fractionation processes are described in EP 412793 B2, EP 773417 B1, EP 780648 B1, EP 807792 B1, EP 932004 A2 and US 2007204652 A1. It is known in such processes to withdraw a first oxygen-enriched residual fraction from the single column, vaporize it and expand at least a part of the vaporized residual fraction in an expansion machine in a work-producing manner in order to generate the refrigeration required to make up the exchange losses and, if appropriate, for product liquefaction. The first residual fraction is given off at the exit pressure of the expansion machine, that is customarily at approximately atmospheric pressure, and can then generally only be used as regeneration gas for an adsorption appliance for purifying the feed air. In SPECTRA processes, a second residual fraction can in this case be taken off from the single column together with the first residual fraction and at least in part vaporized; alternatively, the two residual fractions can be withdrawn at different points of the single column and vaporized separately from one another, for example in different passages of the top condenser of the single column.

One object of this invention is to provide a particularly economically favorable process.

Another object is to provide apparatus to perform the process.

Upon further study of the specification and appended claims, other objects and advantages will become apparent. The figures in parentheses refer to drawings to facilitate an understanding of the invention.

The process comprises a low temperature air fractionation process in which

-   -   feed air (8) is cooled in a main heat exchanger (9) and         introduced into a distillation column system for nitrogen-oxygen         separation (11, 43), which system has at least one single column         (12) for obtaining nitrogen,     -   at least one nitrogen-enriched or oxygen-enriched product stream         (15, 16, 17; 53; 51, 56, 57; 19, 60) is withdrawn from the         distillation column system for nitrogen-oxygen separation and         warmed in the main heat exchanger (9),     -   the single column (12) has a top condenser (13) in which vapor         from the upper region of the single column is at least in part         condensed,     -   a first residual fraction (14, 19) is withdrawn in the liquid         state from the lower region of the single column (12) and at         least in part vaporized in the top condenser (13),     -   at least a first part (20) of the first residual fraction (19)         is expanded downstream of the top condenser (13) in an expansion         machine (21) to produce work and in which     -   a second residual fraction (18, 29) is withdrawn from the lower         or intermediate region of the single column (12), recompressed         (30) and subsequently at least a first part is passed (32) back         to the single column (12), and     -   an improvement wherein         -   a fluid from an external source is passed at least at times             into a liquid tank (70) and         -   at least at times fluid (71) is withdrawn in the liquid             state from the liquid tank (70), vaporized in the main heat             exchanger (9) and is obtained as gaseous additional product             (72, 73), wherein neither the fluid from the external source             nor the gaseous additional product (72, 73) is mixed with a             product from the distillation column system.

The external fluid from the liquid tank is not, as is customary, warmed by means of an external heat exchanger (for example a water bath evaporator or an air-heated evaporator), but in the main heat exchanger in which the feed air for the distillation column system is cooled. By this means, the cold which is present in the external fluid can be recovered for the fractionation process by transmitting it to feed air in the main heat exchanger.

The fluid originates from an “external source”, that is to say not from one of the separation columns of the distillation column system for nitrogen-oxygen separation, or a separation column connected downstream of the distillation column system for nitrogen-oxygen separation. Preferably, it is transported from another installation for generating liquefied gas, for example by means of a tanker. It can be in this case a fluid which has the chemical composition of one of the product streams of the distillation column system for nitrogen-oxygen separation. Preferably, however, the fluid has a different composition from these product streams and consists, for example, of argon or hydrogen. The process according to the invention is thereby suitable, in particular, for supplying factories of the semiconductor industry with industrial gases. These frequently require such a low amount of argon and/or pure oxygen that it is not worthwhile to connect a stage for obtaining argon downstream of the distillation column system for nitrogen-oxygen separation. In addition, the cold of gases such as hydrogen which are not obtained in air fractionation plants is used for air fractionation and thereby the energy consumption of fractionation is decreased.

The “main heat exchanger” is preferably formed by a single heat exchange block. In relatively large plants it can be expedient to implement the heat exchanger by a plurality of trains which are connected in parallel with respect to the temperature course, which trains are formed by construction elements which are separate from one another. In principle, it is possible that the heat exchanger for each of these trains is formed of two or more blocks connected in series.

It is expedient if the pressure of the fluid is not increased between liquid tank and main heat exchanger.

Preferably, the fluid is introduced into the main heat exchanger at a pressure which is not higher than the operating pressure of the liquid tank. The operating pressure of the liquid tank can be constant or fluctuate, for example in the context of maintaining pressure in the tank and the gas withdrawal system. In the event that the operating pressure of the liquid tank fluctuates, here the momentary operating pressure is meant.

It is expedient if the operating pressure of the liquid tank is at least 1 bar above atmospheric pressure, preferably at least 1 bar above the product pressure of the gaseous additional product at which this additional product is given off to an application or a recompressor unit. The operating pressure of the liquid tank is, for example, 2 to 36 bar, preferably 5 to 16 bar. The superatmospheric pressure can be formed by any known measure, for example by charging with a fluid at a corresponding pressure or by pressurizing vaporization.

Further preferred embodiments of the invention include but are not limited to processes wherein:

(a) the second residual fraction (18, 29) is recompressed by means of a cold compressor (30), (b) the mechanical energy generated in the work-producing expansion (21) is at least in part used for recompressing (30) the second residual fraction, (c) a second part of the first residual fraction (19) is not introduced downstream of the top condenser (13) into the expansion machine (21), but is taken off as gaseous impure oxygen product (60) and/or a second part of the second residual fraction is taken off as gaseous impure oxygen product (160) downstream of the recompression (30), (d) an oxygen-containing stream (36) is withdrawn from the single column (12) at an intermediate point and passed (39) to a pure oxygen column (38) and a pure oxygen product stream (41) is withdrawn in the liquid state from the lower region of the pure oxygen column (38), the pure oxygen product stream (41, 56)—if appropriate after pressure elevation (55) in the liquid state is vaporized and warmed against feed air (8) in the main heat exchanger (9) and is finally obtained as gaseous product (57), (e) the first residual fraction (14) is taken off at the bottom of the single column (12), (f) the second residual fraction (18) is taken off from an intermediate point of the single column (12) which is arranged above the bottom, in particular above the point at which the first residual fraction (14) is withdrawn, and (g) the main heat exchanger (9) and the top condenser (13) are formed by apparatuses which are separate from one another.

More detailed descriptions of the above preferred embodiments are incorporated by reference herein from concurrently filed application attorney docket number LINDE-0677 by the present inventor entitled “Process And Device For Low Temperature Air Fractionation” claiming priority of German patent application 102007051184.3 filed Oct. 25, 2007.

The invention, in addition, relates to apparatus comprising

-   -   distillation column system for nitrogen-oxygen separation which         has at least one single column (12) for obtaining nitrogen,     -   a main heat exchanger (9) for cooling feed air (8)     -   a feed air line (11, 43) for introducing cooled feed air into         the distillation column system for nitrogen-oxygen separation,     -   means for feeding a nitrogen-enriched or oxygen-enriched product         stream (15, 16, 17; 53; 51, 56, 57; 19, 60) from the         distillation column system for nitrogen-oxygen separation to the         main heat exchanger (9),     -   a top condenser (13) for the at least partial condensation of         vapour from the upper region of the single column,     -   means for feeding a first liquid residual fraction (14, 19) from         the lower region of the single column (12) to the top condenser         (13),     -   an expansion machine (21) for work-producing expansion of at         least a first part (20) of the first residual fraction (19)         downstream of the top condenser (13),     -   a recompressor (30) for recompressing a second residual fraction         (18, 29) from the lower or intermediate region of the single         column (12)     -   and having means for introducing at least a first part of the         recompressed second residual fraction into the single column         (12), and     -   an improvement wherein:         -   a liquid tank for storage of a fluid from an external source             and         -   means for withdrawing at least at times fluid (71) in the             liquid state from the liquid tank (70) vaporizing it in the             main heat exchanger (9) and obtaining it as gaseous             additional product (72, 73)         -   wherein the device does not have an appliance for mixing the             fluid from the external source or the gaseous additional             product (72, 73) with a product from the distillation column             system.

BRIEF DESCRIPTION OF DRAWING

The invention and also further details of the invention will be described in more detail hereinafter with reference to an exemplary embodiment shown diagrammatically in the drawing.

DETAILED DESCRIPTION OF DRAWING

The distillation column system of the exemplary embodiment has a single column 12 and a pure oxygen column 38. (The invention is likewise applicable to a similar process without pure oxygen column.) Atmospheric air 1 is drawn in through a filter 2 by an air compressor and there compressed to an absolute pressure of 6 to 20 bar, preferably about 9 bar. After it flows through an aftercooler 4 and a water separator 5, the compressed air 6 is purified in a purification device 7 which has a pair of containers filled with adsorption material, preferably a molecular sieve. The purified air 8 is cooled to about dew point in a main heat exchanger 9 and in part liquefied. A first part 11 of the cooled air 10 is introduced via a throttle valve 51 into the single column 12. Said cooled air is fed in preferably some practical or theoretical plates above the bottom.

The operating pressure of the single column 12 (at the top) is 6 to 20 bar, preferably about 9 bar. Its top condenser is cooled by a second residual fraction 18 and a first residual fraction 14. The first residual fraction 14 is taken off from the bottom of the single column 12, the second residual fraction 18 from an intermediate point some practical or theoretical plates above the air infeed or at the same height as this.

As the main product of single column 12, gaseous nitrogen 15, 16 is taken off at the top, warmed in the main heat exchanger 9 to approximately ambient temperature and finally taken off via line 17 as pressurized gaseous product (PGAN). A part 53 of the condensate 52 from the top condenser 13 can be obtained as product liquid nitrogen (PLIN); the remainder 54 is applied as reflux to the top of the single column.

The second residual fraction 18 is vaporized in the top condenser 13 at a pressure of 2 to 9 bar, preferably about 4 bar, and flows in the gaseous state via line 29 to a cold compressor 30 in which it is recompressed to approximately the operating pressure of the single column. The recompressed residual fraction 31 is cooled back to column temperature in the main heat exchanger 9 and finally fed back via line 32 to the single column 12 at the bottom.

The first residual fraction 14 is vaporized in the top condenser 13 at a pressure of 2 to 9 bar, preferably about 4 bar, and flows in the gaseous state via line 19 to the cold end of the main heat exchanger 9. A first part 20 of the first residual fraction is withdrawn again (line 20) at an intermediate temperature. A second part remains in the main heat exchanger 9, is warmed there again to approximately ambient temperature and leaves the installation via line 60 as gaseous impure oxygen product (GOX-Imp.). The first part 20 of the first residual fraction is expanded to about 300 mbar over atmospheric pressure so as to produce work in an expansion machine 21 which is constructed in the example as a turbo expander. The expansion machine is mechanically coupled to the cold compressor 30 and a braking appliance 22 which, in the exemplary embodiment, is formed by an oil brake. The expanded first residual fraction 23 is warmed in the main heat exchanger 9 to approximately ambient temperature. The warm first residual fraction 24 is blown off to atmosphere (line 25) and/or used as regeneration gas 26, 27 in the purification device 7, if appropriate after heating in the heating appliance 28. Alternatively, or in addition, an impure oxygen product can be branched off from the recompressed second residual fraction 31 and warmed in the main heat exchanger 9 to approximately ambient temperature.

An oxygen-containing stream 36 which is essentially free of low volatility impurities is taken off from an intermediate point of the single column 12 in the liquid state, which intermediate point is arranged 5 to 25 theoretical or practical plates above the air infeed. The oxygen-containing stream 36 is, if appropriate, subcooled in a bottoms evaporator 37 of the pure oxygen column 38 and applied to the top of the pure oxygen column 38 via line 39 and throttle valve 40. The operating pressure of the pure oxygen column 38 (at the top) is 1.3 to 4 bar, preferably about 2.5 bar.

The bottoms evaporator 37 of the pure oxygen column 38 is in addition cooled by means of a second part 42 of the cooled feed air 10. The feed air stream 42 is in this case at least in part, for example completely, condensed and flows via line 43 to the single column 12 where it is introduced approximately at the height of the infeed of the remaining feed air 11.

From the bottom of the pure oxygen column 38, a pure oxygen product stream 41 is withdrawn in the liquid state, brought by means of a pump 55 to an elevated pressure of 2 to 100 bar, preferably about 12 bar, passed via line 56 to the cold end of the main heat exchanger 9, vaporized there at the elevated pressure and warmed to about ambient temperature and finally obtained via line 57 as gaseous product (GOX-IC).

The overhead gas 58 of the pure oxygen column 38 is admixed to the expanded first residual fraction 23. Via a bypass line 59, if appropriate a part of the feed air is passed for pump protection of the cold compressor 30 to the inlet thereof (anti-surge control).

If required, a liquid oxygen can be withdrawn as liquid product from the installation upstream and/or downstream of the pump 55 (not shown in the drawing). In addition, an external liquid, for example liquid argon, liquid nitrogen or liquid oxygen from a liquid tank can be vaporized in the main heat exchanger 9 in indirect heat exchange with the feed air (not shown in the drawing).

A liquid tank 70 is filled with liquid argon as “fluid” from time to time from a tanker. The fluid is introduced at about 12 bar, the operating pressure of the liquid tank. Liquid fluid is withdrawn continuously via line 71 at about 12 bar, vaporized at this pressure in the main heat exchanger 9 and warmed and finally taken off as gaseous additional product via lines 72 and 73.

In addition, a further stream 74 of the liquid pressurized fluid can be withdrawn from the liquid tank 70, vaporized in an evaporator 75 which is heated by means of an external heat carrier (for example atmospheric air or water) and added via line 76 to the gaseous additional product. The evaporator 75 can, however, also be used for emergency supply in the event of loss of the main heat exchanger 9. The flow rates are set by means of the valves 77 and 78.

The process according to the invention and the corresponding device can be used particularly expediently in the semiconductor industry or in the pyrogenic silicic acid production which require not only nitrogen but also impure oxygen and if appropriate pure oxygen as products.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding DE application No. 102007051183.5, filed Oct. 25, 2007, as well as concurrently filed the disclosure relating to claims 5-11 in particular, incorporated by reference herein.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for low temperature air fractionation in which feed air (8) is cooled in a main heat exchanger (9) and introduced into a distillation column system for nitrogen-oxygen separation (11, 43), which system has at least one single column (12) for obtaining nitrogen, at least one nitrogen-enriched or oxygen-enriched product stream (15, 16, 17; 53; 51, 56, 57; 19, 60) is withdrawn from the distillation column system for nitrogen-oxygen separation and warmed in the main heat exchanger (9), the single column (12) has a top condenser (13) in which vapor from the upper region of the single column is at least in part condensed, a first residual fraction (14, 19) is withdrawn in the liquid state from the lower region of the single column (12) and at least in part vaporized in the top condenser (13), at least a first part (20) of the first residual fraction (19) is expanded downstream of the top condenser (13) in an expansion machine (21) to produce work and in which a second residual fraction (18, 29) is withdrawn from the lower or intermediate region of the single column (12), recompressed (30) and subsequently at least a first part is passed (32) back to the single column (12) characterized in that a fluid from an external source is passed at least at times into a liquid tank (70) and at least at times fluid (71) is withdrawn in the liquid state from the liquid tank (70), vaporized in the main heat exchanger (9) and is obtained as gaseous additional product (72, 73), wherein neither the fluid from the external source nor the gaseous additional product (72, 73) is mixed with a product from the distillation column system.
 2. A process according to claim 1, characterized in that the fluid (71) is not increased in pressure between liquid tank (70) and main heat exchanger (9).
 3. A process according to claim 1, characterized in that the fluid (71) is introduced into the main heat exchanger (9) at a pressure which is not higher than the operating pressure of the liquid tank (70).
 4. A process according to claim 1, characterized in that the operating pressure of the liquid tank (70) is at least 1 bar above atmospheric pressure.
 5. A process according to claim 1, characterized in that the second residual fraction (18, 29) is recompressed by means of a cold compressor (30).
 6. A process according to claim 1, characterized in that mechanical energy generated in the work-producing expansion (21) is at least in part applied to recompressing (30) the second residual fraction.
 7. A process according to claim 1, characterized in that a second part of the first residual fraction (19) is not introduced downstream of the top condenser (13) into the expansion machine (21), but is taken off as gaseous impure oxygen product (60) and/or a second part of the second residual fraction is taken off as gaseous impure oxygen product (160) downstream of the recompression (30).
 8. A process according to claim 1, characterized in that an oxygen-containing stream (36) is withdrawn from the single column (12) at an intermediate point and passed (39) to a pure oxygen column (38) and a pure oxygen product stream (41) is withdrawn in the liquid state from the lower region of the pure oxygen column (38), the pure oxygen product stream (41, 56), after optional pressure elevation (55) in the liquid state, is vaporized and warmed against feed air (8) in the main heat exchanger (9) and is thus obtained as gaseous product (57).
 9. A process according to claim 1, characterized in that the first residual fraction (14) is taken off at the bottom of the single column (12).
 10. A process according to claim 1, characterized in that the second residual fraction (18) is taken off from an intermediate point arranged above the bottom of the single column (12), in particular above the point at which the first residual fraction (14) is withdrawn.
 11. A process according to claim 1, characterized in that the main heat exchanger (9) and the top condenser (13) are separate from one another.
 12. Apparatus for low temperature air fractionation having distillation column system for nitrogen-oxygen separation which has at least one single column (12) for obtaining nitrogen, a main heat exchanger (9) for cooling feed air (8) a feed air line (11, 43) for introducing cooled feed air into the distillation column system for nitrogen-oxygen separation, means for feeding a nitrogen-enriched or oxygen-enriched product stream (15, 16, 17; 53; 51, 56, 57; 19, 60) from the distillation column system for nitrogen-oxygen separation to the main heat exchanger (9), a top condenser (13) for the at least partial condensation of vapor from the upper region of the single column, means for feeding a first liquid residual fraction (14, 19) from the lower region of the single column (12) to the top condenser (13), an expansion machine (21) for work-producing expansion of at least a first part (20) of the first residual fraction (19) downstream of the top condenser (13), a recompressor (30) for recompressing a second residual fraction (18, 29) from the lower or intermediate region of the single column (12) and having means for introducing at least a first part of the recompressed second residual fraction into the single column (12), characterized by a liquid tank for storage of a fluid from an external source and means for withdrawing at least at times fluid (71) in the liquid state from the liquid tank (70) vaporizing it in the main heat exchanger (9) and obtaining it as gaseous additional product (72, 73) wherein the device does not have an appliance for mixing the fluid from the external source or the gaseous additional product (72, 73) with a product from the distillation column system. 